The effect of nitrous oxide in 66 per cent concentration as commonly used in surgical anaesthesia, on differential white cell counts was studied during and after the operations lasting longer than two hours in two groups of patients one of which did not receive nitrous oxide. Differential counts taken from both groups showed similar changes e.i. a significant increase in neutrophil percentage and a significant decrease in lymphocyte percentage, during and after surgery. There were no significant changes in the percentage of eosinophils, monocytes or basophils. It was concluded that nitrous oxide on itself as used in surgical anaesthesia, could not be held responsible for the changes seen in the differential white cell counts.